A typical organic electroluminescent element (hereinafter also referred to as an organic EL element) is composed of a cathode, an anode, and a luminous layer containing a luminous compound and disposed therebetween. An electric field applied to such a light-emitting element recombines holes injected from the anode with electrons injected from the cathode in the luminous layer to generate excitons, which are deactivated with luminescence (fluorescence and/or phosphorescence). The organic EL element can emit light by such a mechanism. The organic EL elements are completely solid elements each including submicron films composed of organic materials, the films being disposed between electrodes and being capable of emitting light at an applied voltage of about several volts to several tens of volts. Such organic EL elements have great potential in applications to next-generation flat panel displays and illumination devices.
Since Princeton University reported an organic EL element by phosphorescence from the excited triplet state, phosphorescent materials at room temperature have been extensively investigated for practical use.
The luminescence efficiency of organic electrophosphorescent elements, in principle, can be about four times higher than that of traditional organic electrofluorescent elements, and world-wide studies and developments have been conducted on phosphorescent materials, layer configurations, and electrodes included in light-emitting elements. Especially, tremendous expectations have been placed on the development of novel materials for enhancing the performance of the organic EL elements.
As described above, the phosphorescent mechanisms have significantly high potential. Unlike organic EL devices utilizing emission of fluorescence, however, the organic phosphorescent devices should satisfy the following technical requirements for the efficiency and the service life of organic EL devices through control of the central position of light emission, particularly control of recombination of holes with electrons inside the luminous layer to attain stable light emission.
One of known solutions to such problems is multi-layered elements each including a laminate of a luminous layer, a hole transporting layer adjacent to an anode, and an electron transporting layer adjacent to a cathode. The luminous layer is composed of a mixed layer of a luminous host and a phosphorescent compound as a luminous dopant in many cases.
As for the materials, development of novel materials has been tremendously expected for enhancing the performance of organic EL elements.
A variety of materials for organic EL elements have been reported. For example, it is already known that dibenzofuran or dibenzothiophene compounds having specific substituents are useful as materials for organic EL elements in view of heat resistance and reduced defects of pixels (for example, see Patent Literature 1, 2, 3, and 4).
Unfortunately, high luminescence efficiency in organic EL elements requires homogeneous dispersion of a dopant as a luminous material for a reduction in concentration quenching caused by agglomeration of the dopant or quenching caused by interaction between excitons. It has been found that organic EL elements containing the compounds described in these documents as luminous hosts have insufficient luminescence efficiency and emission lifetimes, in regions containing particularly high concentration of dopants, and additional techniques are required to attain sufficient luminescence efficiency and prolonged emission lifetimes.
The performance of organic EL elements highly depends on the morphology of thin films. Typically, the organic EL elements suitably include amorphous thin films. Microcrystals present in a thin film function as nuclei to grow into crystals in the film during a driving mode and storage over time of the organic EL elements. These crystals increase grain boundaries into which an electric field is concentrated, resulting in unsatisfactory electrical characteristics and short service lives of the organic EL elements.
It has been reported that the control of molecular orientation even in amorphous films is important to control the electrical and optical characteristics of organic EL elements. For example, the results of detailed analysis of the molecular orientation (for example, see Non-Patent Literature 1) suggest that charge transportation is significantly influenced by the molecular orientation in amorphous films. Molecules in amorphous films are normally oriented in different directions. Such different directions of orientation reduce interaction between the molecules and preclude movement of carriers, leading to an increased driving voltage.
Requirements for an increase in area of organic EL elements, a reduction in cost, and higher productivity lead to expectations on wet processes. In addition, the wet processes can form films at lower temperature compared to vacuum processes to reduce damage of an underlying organic layer and increase the luminescence efficiency and the service life of the organic EL elements.
Bottlenecks in preparation of organic EL elements by wet processes are the film forming characteristics of the luminous host included in a luminous layer and an electron transporting material deposited on the luminous layer, and the solubilities of these materials in solvents for preparing coating solutions. The present inventors have found that traditional luminous hosts and electron transporting materials have low solubilities in solvents and solution stability at a practical level, and should be further technically improved.
In conclusion, such traditional materials cannot produce high-performance organic EL elements, and novel materials have been demanded for enhancing the performance of organic EL elements. Such materials should preferably be suitable for preparation of organic EL elements by wet processes.